1. Field of the Invention
The present invention relates to a CMOS image sensor, and more particularly to a CMOS image sensor and method for manufacturing the same, in which a diffusion region of a photo diode is disposed apart from an isolation layer, and thereby a dark current may be reduced.
2. Description of the Related Art
In general, an image sensor is a semiconductor device for converting an optical image into an electrical signal, and is generally classified into either a charge coupled device (CCD) or a complementary MOS (CMOS) image sensor.
The CCD is a device constructed in such a manner that each MOS capacitor is adjacently disposed to each other, and that charge carriers are stored on any one of the MOS capacitors and then transferred to another MOS capacitor next to the MOS capacitor stored by the charge carrier. The CCD has various disadvantages, such as complicated drive mode, much power consumption, complicated manufacturing process derived from many photo processes and so forth. Additionally, the CCD has a disadvantage in that it is difficult to make a product compact, due to difficulty in integrating various circuits such as a controlling circuit, a signal processing circuit, analog/digital converting circuit, etc., into a chip for the CCD.
Currently, as a next generation image sensor for overcoming the disadvantages of the CCD, attention is focused on CMOS image sensors. The CMOS image sensor is a device employing a switching mode of forming MOS transistors as many as the number of unit pixels on a semiconductor substrate using CMOS technology for a controlling circuit, a signal processing circuit, etc as a periphery circuit, and sequentially detecting outputs of each unit pixel by means of the MOS transistors. That is, the CMOS image sensor sequentially detects electrical signals of each unit pixel in a switching mode to realize an image through formation of a photo diode and a MOS transistor within a unit pixel.
The CMOS image sensor has advantages such as low power consumption, simple manufacturing process resulting from decreased photo processes, etc., because it makes use of such CMOS manufacturing technology. In addition, the CMOS image sensor has an advantage in that it is easier to make a product compact by integration of a controlling circuit, a signal processing circuit, an analog/digital converting circuit, etc. into a chip for the CMOS image sensor. For this reason, the CMOS image sensor is presently broadly used in various applications, such as digital still cameras, digital video cameras and so forth.
FIG. 1 shows a circuit for a unit pixel of a general CMOS image sensor. As shown in FIG. 1, the unit pixel 100 of the CMOS image sensor includes a photo diode 110 as a photoelectric transformation section and four transistors. The four transistors may include a transfer transistor 120, a reset transistor 130, a drive transistor 140 and a select transistor 150. An output terminal OUT of the unit pixel 100 may be connected with a load transistor 160. Herein, reference FD may be a floating diffusion region, reference Tx may be a gate voltage of the transfer transistor 120, reference Rx may be a gate voltage of the reset transistor 130, reference Dx may be a gate voltage of the drive transistor 140, and a reference Sx may be a gate voltage of the select transistor 150. Reference VDD is an input voltage, reference Vb is a base voltage, and reference GND is a ground.
FIG. 2 shows a layout of a unit pixel of the conventional CMOS image sensor. As shown in FIG. 2, in a unit pixel 100, an active region may be a region defined by a bold solid line and an isolation region may be a region outside the active region in which an isolation layer (not shown) may be formed. The gates 123, 133, 143 and 153, respectively of the transfer transistor 120, reset transistor 130, drive transistor 140 and select transistor 150 may be disposed as to be across an upper portion of the active region. Also, a region indicated by a dotted line may be a region which is exposed in an opening of a photoresist (not shown) as a masking layer against ion implantation upon the ion implantation process for forming a diffusion region of the photo diode and which may be defined wider than an active region of the photo diode PD. The reference FD may be a floating diffusion region.
FIG. 3 is a structural cross-sectional view showing the photo diode portion of the unit pixel taken along a line A-A of FIG. 2. As shown in FIG. 3, a P− type epitaxial layer 11 may be formed on a P++ type semiconductor substrate 10. To define an active region of the semiconductor substrate 10, an isolation layer 13 may be formed on a portion of the epitaxial layer 11 for an isolation region of the semiconductor substrate 10. An n− type diffusion region 111 and a P0 type diffusion region 113 of the photo diode PD may be formed on a portion of the epitaxial layer 11 for a photo diode region of the active region, the P0 type diffusion region 113 being positioned on the n− type diffusion region 111.
The conventional CMOS image sensor 100 with such construction typically has a defect such as degradations of a performance of the device and an electric charge storing capacity, due to an increase of dark current.
Dark current may be generated by electrons transferred to the floating diffusion region from the photo diode in a state that lights are not yet incident to the photo diode. It has been reported that dark current has been caused generally from various kinds of defects, such as a dangling bond and so forth disposed on a neighboring portion adjacent to the surface of the semiconductor substrate, a boundary portion of the isolation layer and the P0 type diffusion region, a boundary portion of the isolation layer and the n− type diffusion region, a boundary portion of the P0 type diffusion region and the n− type diffusion region, the p0 type diffusion region and the n− type diffusion region. Dark current may cause considerable problems such as degradations of a performance and an electric charge storing capacity in the CMOS image sensor under a circumstance of low illumination.
Accordingly, the conventional CMOS image sensor has used both the P0 type diffusion region and the n− type diffusion region for the photo diode in order to reduce dark current generated especially from the neighboring portion adjacent to the surface of a silicon substrate.
However, the conventional CMOS image sensor has been greatly affected by dark current generated at the boundary portions of the isolation region and the P0 type diffusion region, and the P0 type diffusion region the n− type diffusion region.
More particularly, as shown in FIG. 3, when patterns (not shown) of a photoresist as a mask layer against an ion implantation for forming the n− type diffusion region 111 and the P0 type diffusion region 113 of the photo diode PD may be formed on the semiconductor substrate 10, the whole active region for the photo diode PD is exposed in an opening of the photoresist patterns. In such state, when impurities for the n− type diffusion region 111 and the P0 type diffusion region 113 are ion-implanted in the active region of the photo diode PD, the impurities for the n− type diffusion region 111 and the P0 type diffusion region 113 are also ion-implanted to the boundary portion between the active region and the isolation layer 13 of the photo diode PD.
Thus, damages by the ion implantation of impurities may be caused at the boundary portion between the n−/P0 type diffusion regions 111 and 113 and the isolation layer 13, further generating defects. The defects may cause a generation of electric charge or hole carriers, and also provide places for re-binding of the electric charges and the halls. Consequently, leakage current of the photo diode and therefore dark current of the CMOS image sensor are increased.
As described above, the conventional CMOS image sensor may have a construction in which impurities for forming the diffusion region of the photo diode are also ion-implanted on the boundary portion between the isolation layer and the active region for the photo diode when the impurities are ion-implanted for forming the diffusion region of the photo diode. Thus, the conventional CMOS image sensor may have a limit in increasing characteristics of the device because it may be difficult to restrict dark current generated at the boundary portion between the isolation layer and the active region for the photo diode, and to maintain device characteristics between the pixels constant.
Meanwhile, Korean Laid-Open Patent Publication Nos. 2003-42303 and 2003-42308 disclose a method for reducing dark current of CMOS image sensor by performing an ion implantation of impurities to an active region for a photo diode, which, however, do not present a solution to restrict an increase of dark current by preventing impurities from being ion-implanted to the boundary portion between an isolation layer and an active region for a photo diode.
Also, U.S. Pat. Nos. 6,486,521 and 6,462,365, assigned on their face to Omnivision Technologies Inc., entitled “Active Pixel having Reduced Dark Current in a CMOS Image Sensor,” disclose a method for restricting an increase of dark current due to a dangling bond at the surface of a photo diode, in which a transparent insulating layer such as an oxide layer is formed on the surface of the photo diode as a passivation layer. However, the methods do not also present a solution to restrict an increase of dark current by preventing impurities from being ion-implanted to the boundary portion between the isolation layer and the active region for the photo diode.